BackgroundProcessor is not wasm-web compatible

[benthecarman]

Because BackgroundProcessor::start uses std::thread and std::time::Instant this function is unable to be called in a wasm runtime. It would be great if these were able to be abstracted out or a separate version that didn't require these was available so everything could be run in the browser.

For now our solution is just copy-pasting the define_run_body macro into our own code and seeing if we can get that to work. However, this is not the best alternative as we'd like to just use what rust-lightning has so we don't need to check if this macro has changed every time we update the dependency.

[TheBlueMatt]

I think we can make the futures feature version no-std compatible (insofar as the actual futures runtime can generate a future for a timer) by re-using the existing sleep-future-fetcher to do all the time checks. This would let JS folks spawn a JS timer and then pass it back through as a JsFuture (I think).